fix: errors in F004 (division by zero) and L003 (bugs on MacOS) fixed

Description

As it was discovered during the use of postgres-checkup in some larger setups, in some cases when a table is empty and its size is 0, it can cause the error "division by zero" in F004.

To mitigate this issue, in some places where this error might occur, additional checks were added.

For report L003, it was found that bash snippet with multiplication (let "mulvar=$var * 8192") does not work well on MacOS. It was replaced by mulvar=$((var * 8192)).